This invention relates to hydraulic valves and to systems including such valves.
Hydraulic systems are used extensively to control the engagement of a clutch in a gearbox connecting the output shaft of an engine to its driven machinery, especially in heavy load applications, such as marine engines or an earth moving plant. The hydraulic system includes an electrically-operated or mechanically-operated valve, which enables or prevents flow of hydraulic fluid from a pump to an hydraulic clutch actuator. Where the gearbox has two clutches, such as for forward and reverse drive, two valves are used to supply hydraulic fluid to one or the other of the clutch actuators. The valves are arranged in such a way that only one can be open at any time, so as to prevent both clutches being engaged at the same time. Such valves can only adopt an open or closed state. This can be a disadvantage since it can be useful to be able to control the application of a variable hydraulic pressure to the clutch actuator, to enable it to be gradually engaged or disengaged, or to be partially engaged. There are other applications where it can be an advantage to be able to control the application of a variable hydraulic pressure to an hydraulic actuator.